Diamonds are formed under high pressure deep within the Earth's mantle, generally more than 150 km below the crust, and ascend to the surface when magma rises and erupts in small but violent volcanoes. The volcanic action creates carrot-shaped ‘pipes’ filled with volcanic rock forming diamond ore, such as kimberlite or lamproite. In addition, diamonds may be found in alluvial or marine terrace deposits. Kimberlite ores, for example, are ultrabasic and weather readily. In tropical climates, kimberlite weathers to ‘yellow ground’ which is predominantly comprised of clays. In temperate climates, weathering is less severe, but clays are still the predominant weathering product. The ore also includes gangue minerals, such as olivine ((Mg,Fe)2SO4), garnet ((Fe,Al)(SiO4)3), diopside ((Ca,Mg,Cr)SO4), magnesian ilmenite ((Fe,Mg)TiO3), enstatite ((Mg,Fe)2Si2O6), chromite ((Fe,Mg)O(Fe,Al,Cr)2O3), phlogopite (KMg3AlSi3O8(F,OH)2, and serpentine (Mg6(OH)8Si4O10).
An exemplary method of processing fresh diamond ores begins with crushing. An ore may be crushed initially to about or below 250 mm by means of a gyratory crusher and then to about 75 mm using a cone crusher. Then another stage of crushing may be accomplished by means of a high-pressure roller mill to further reduce the size below 25 mm. The crushed ore may be scrubbed in a tumbler in the presence of water, and the fines generated during crushing and scrubbing may be removed by means of a screen that may have, for example, 2 mm apertures.
The crushed ore particles that are finer than 25 mm but larger than 2 mm may be sent to gravity separation devices such as jig or heavy medium cyclones to remove light minerals from heavies. The heavy minerals, which include diamond, may be sent to X-ray sorters that are designed to spot diamonds by fluorescence and then activate air jets that sweep diamonds from a belt into a collector. The reject from the X-ray sorters may be sent to greased tables (or belts), which are covered with thick layers of oily substances such as lubricating oils, paraffin oils and other materials where some of the diamonds may be retained.
Material finer than about 2 mm may be screened at about 0.5 mm, and the screen overflow, i.e., material between about −2+0.5 mm, may be subjected to flotation. In this process, it is contemplated that the particles may be suspended in water and air bubbles may be injected into the suspension. The air bubbles may pick up some of the diamonds selectively on the surface and the resulting bubble-particle aggregates rise to the surface of the water. As it is well known to those skilled in the art, flotation may be conducted with the addition of reagents such as frothers that may increase the abundance of gas bubbles or control gas bubble size distribution. It should be appreciated, however, that the ore sizes discussed above, are exemplary for the purposes of background discussion and should not be construed as limiting the processes discussed herein.
Without being bound to any particular theory, it is reported that the above described diamond separation processes, i.e. the greased tables and diamond flotation, function due to the principle of hydrophobic interaction. Hydrophobic interaction refers to the attraction between hydrophobic substances in water.
Comprised mainly of carbon atoms, diamond is relatively nonpolar and naturally hydrophobic. The diamonds may also have water contact angles of greater than about 60° as measured by the sessile drop technique, including all increments and values therein, such as 65°, 75°, etc. Accordingly, diamond crystals may stick to the grease on the greased table and to the air bubbles, as both the grease and the air bubbles are nonpolar and hydrophobic, allowing the separation of the diamonds from the hydrophilic gangue minerals during the processing of kimberlite and lamporite ores.
It has been reported that diamonds, such as, for example, alluvial diamonds, and particularly those from marine terraces, are frequently coated with films of mineral salts. The films may be formed by the adsorption of various ions such as Fe3+, Mg2+, silicate, carbonate, etc., on the surface of the diamond when, for example, the diamonds are placed in water that has contacted kimberlite ore or other ore material. This may cause the diamonds to become less hydrophobic and more water wettable (i.e., hydrophilic). Slime coatings may also occur upon the contact of diamonds with process water during industrial separation. Due to their mainly ionic composition, the slime coatings may also render the diamond surfaces less hydrophobic and more water wettable (i.e., hydrophilic). For example, the diamonds may have surface contact angles of less than about 60° after contacting ore slurry, including all ranges and increments therein such as 45°, 59°, etc., as measured by the sessile drop technique. These phenomena may be referred to as ‘secondary hydrophilization’. Secondary hydrophilization may render the yield of diamond separation processes less than practically feasible.